Where did Mark Lynas get the figures for the various public transport modes? And would he equate light rail (as in the Sheffield tram system etc) to the underground? I live in Australia where it is hard to find these figures.David Meiklejohn (via email)

Mark Lynas: These are UK government figures - and yes I would equate light rail with underground. Obviously if you want to be really nit-picky the carbon use of light rail depends on the generating mix of the electricity it uses - and your juice in Australia may include a higher coal component than ours does in the UK. But for most people this won't be a significant part of their footprint.

All for one

In your calculator, some of the figures are per person, and some per household. Doesn't that make it impossible to compare my results with national averages per person? There are three adults in my house, so should I divide the heating costs by three? Posted by Ortho

Mark Lynas: Yes, you should. It takes exactly the same energy to heat a room for one person as it does to heat it for three. Obviouslyas there are three of you in the house you may be heating rooms that would not be used if there was only one person, but overall the impact is still much less than if each adult ran their own house. That's why the quickest way to reduce your carbon footprint from domestic energy use is to house-share.

Eating your greens

The carbon footprint appears not to take into account the importance of adopting a low-carbon diet, eg veganism. Eating is something we do everyday and the vegan diet is one that I have chosen not only for animal rights issues but for environmental reasons. The consumption of animal products is so often missed off these carbon footprint calculators but meat based diets are so environmentally destuctive (not to mention unhealthy). Posted by VeganBee

Mark Lynas: There is a disturbing tendency for vegans to argue that a change in eating habits can solve all the world's problems in one go. Having said that, there's no doubt that diets involving high consumption of meat and dairy products are more energy intensive than those which include only fruit and vegetables. But there are many different factors to consider here: a grass-fed Welsh lamb might have a lower carbon footprint (and be a good deal more succulent) than soya mince substitute imported from former rainforest land in the Amazon.

Ah, but I hear you reply - don't I realise that most UK-reared animals are fed with a good deal more soya in their diet than you would eat directly as a vegan? That's true too - so a responsible meat-eater will ask the farmer at their local farmers' market whether most of the animal's feed has also been local. As long as this is considered I would argue that eating small amounts of meat on an occasional basis (as a treat) is perfectly compatible with a low-carbon lifestyle.

It doesn't add up

I'm a bit puzzled by the two calculators for long haul flights. For a return flight London Delhi I get two quite different readings of either 1.88 or 4.00. Obviously I would like to take the lower reading, but my conscience tells me to check honestly! Posted by baazigar

Mark Lynas: This is probably a good point to admit that I don't know the definitive answer. Disputes over figures bedevil this whole calculating process, as you've probably noticed. There is no single standardised figure for ferry travel, for example, so I really can't tell you with any certainty whether you're better off flying to India or going there on a ship. It really ought to be up to some official agency like the government to sort this out for us, but so far they haven't.

I suspect the discrepancy in your figures comes because of different underlying calculating assumptions used by the websites - particularly with regard to the "multiplier" (factoring in the additional warming effect of non-CO2 gases released by aircraft), seating and occupancy, fuel use and so on. If you go with the higher figure though then you know you're cutting yourself some slack!

Boxing clever?

What are your views on organic fruit-and-veg boxes? I live in a city in the West Midlands, where the nearest shop, which has organic food for sale, is a large supermarket. Often it isn't even clear whether their organic offerings haven't been flown in. Posted by BienH

Mark Lynas: My verdict, young pilgrim, is that as a general rule local trumps organic, but local and organic is the best of all. It sounds like you are in the happy situation of having local and organic veg available from a nearby box scheme, which is great. I suggest you use your local shop (not an evil supermarket) for additional items - if you need to buy ketchup for your organic chips, for instance, or if you ever tire of swedes.

Power point

If I get my electricity from a supplier who sources only from renewable sources - eg Good Energy - does that mean that it doesn't count towards my carbon footprint? Posted by mkinghan

Mark Lynas: As I indicate in the article and the book, this is not a simple issue to quantify. The renewable electricity provider Good Energy does guarantee to match your consumption with 100% renewable power, so I think it is fair to count electricity from them as carbon neutral as a result. But there is an issue of scalability here: if the entire country signed up to Good Energy overnight, would there be enough renewable power for everyone? Obviously not. But this does at least push the market in the right direction.

Even more confusingly, all green tariffs are different: you need to be sure that any renewable power you buy is 'additional' to what would have been generated anyway because of the government's mandatory investment scheme, called the renewables obligation. Otherwise you might simply be paying a premium for something the company had to do anyway. My advice is to contact your green tariff provider with specific questions about what the renewable content of their electricity is.

Small footprints

Why do you only count adults in working out per person emissions and not children? Our children have a bedroom each, heated and lit, and spend more time than us in the living room, watch more TV, play more computer games.Posted by AlanClarke

Mark Lynas: The issue of kids is a thorny one, and the various proponents of personal carbon allowances (carbon rations) take different views on it. My opinion, for what it's worth, is that as long as children are dependent on you as a parent for food, shelter and other support, then they are also dependent on you for a carbon allowance, and shouldn't have their own carbon footprints.

This is, of course, based on the assumption that you are making the final decision about household energy use - such as the switching off of hairdryers, computer games, and so on, on their behalf. But maybe that isn't the case?

Every breath you take

What's the carbon footprint involved in breathing? We've been hearing a lot about methane of late, but what about the steady stream of carbon dioxide from our noses? Posted by kimberlee

Mark Lynas: A common misconception. (Unless you're joking, in which case I apologise for the sense of humour failure.) I'm only talking about the use of fossil carbon, which increases the net annual accumulation in the atmosphere when dug up from oil and coal deposits and burned.

Carbon that we metabolise in the body comes from solar-generated sources like plants (unless you're drinking motor oil), which took the carbon in from the atmosphere in the first place during photosynthesis. As a result it doesn't add to the total - you're not combusting fossil fuels directly in your body.

You may be combusting them indirectly in your food consumption - through fertiliser use, food miles, and so on - but this is accounted for (admittedly in a rather generalised form) in your carbon calculation.

Working total

Surely we are all responsible for the amount of carbon emitted by our employers in providing us with computers, lighting, heating (and for those that don't work in the public sectors, cooling). I wonder if what, in the interest of arriving at a real life yearly total, the figures for that would be? David Thomas (via email)

While those who work outside their home do not have their activities there included in their carbon footprint (except for journey to work), I, who work from home, can only get a footprint measurement which includes both home and work output. Posted by GrumpyR

Mark Lynas: My view is that carbon emissions resulting from one's role as an employee are not a personal responsibility and should come under an employer's corporate carbon budget. Since companies are increasingly doing these calculations too, this is important to eliminate double-counting.

Having said that, we all have a role in making our workplaces greener - and opting for the train to Paris rather than the plane, for example, may be something employers readily agree to.

With regard to working at home - I have the same issue in my own life, and find that the best way to draw the line is in the same way you do for the dreaded tax return. As a homeworker, you presumably count a portion of your expenses against tax as work-related. Similarly, you could count some activities towards your "work" carbon budget rather than your personal one.

Location, location, location

I'd be interested to know whether you have considered "correcting" the carbon footprint for the environment of the user. I think that this would be considered fair given that someone in the Western Isles is going to spend considerably more than someone on the English Riviera in heating costs. I also remember a university lecturer once saying that 70% of the food eaten in Glasgow goes directly to heat production, given the cool climate, a figure that drops dramatically in more temperate climes.

There is also the transport factor with city dwellers benefiting from better public transport than those living in the countryside. I feel that it may be fairer to set a carbon cost of living for each area at a fair rate and then allow people to have a more realistic target to aim for .... Posted by CPCampbell

Mark Lynas: A fair enough point, but this is potentially a Pandora's box. In the international Kyoto negotiations, Australia argued (successfully but outrageously) that it should be allowed to increase emissions because its cities are so far away from each other that people have to travel a lot. You're making a similar argument for rural dwellers.

There may be some truth in it, but once this box is opened, everyone quickly argues that they should be seen as a special case. Would you argue that people in cities should have a lower allocation because they don't need to use cars so much? Better in my view for us all to be treated equally than to spend years arguing over special circumstances.

· Mark Lynas is the author of Carbon Counter, published by HarperCollins. To buy a copy of the book for £4.99 visit the Guardian Bookshop.